Treatments for detoxification of polychlorinated biphenyls are accelerating globally. We have developed an environmentally sound chemical detoxification plant for polychlorinated biphenyls using a combination of photochemical dechlorination (P. C. D.) and catalytic dechlorination (C. D.). First, a rate equation was modeled based on the results of experiments on P. C. D. On the other hand the C. D. by use of palladium carbon catalyst (Pd/C) was considered to remain chlorines at the ortho positions of PCB rings and therefore to be unsuitable for the degradation of PCB homologues such as tetrachlorobiphenyls and pentachlorobiphenyls. Additionally, for an initial concentration of PCB of more than 1, 000 ppm, the crystallization of NaCl, one of the by-products was considered to inhibit the homogeneous dispersion of particles of catalyst in the solvent (isopropyl alcohol). Performance tests with the combined system were done, and finally a rate constant of 0.132 min(-1) was obtained, which was greater than the target value of 0.102 min(-1) for this reaction. In conclusion the validity of this combined system for commercial-scale plants was confirmed.